Hi
Liked the tutorial. One additional thing that you might want to reference at this point

How sort actually works internally doesn't really matter. That it is about as fast as you are likely to get; that it is built-in to language so always available; and that it understands Perl internals; does.

is that the internals of the Perl sort changed from quicksort to a mergesort between 5.6 and 5.8.
Although the main change is probably to defend against worst-case O(N**2) behaviour, the user can be bitten by this "feature"
From CPAN Sort.pm

A stable sort means that for records that compare equal, the original input ordering is preserved. Mergesort is stable, quicksort is not.

This means if you use 5.6, and your comparator returns 0 for 2 items that are in fact different, the input order and the output order of those items may change. This behaviour burned me recently; others may want to know about it.